Confirmation bias, the triplet number test where the rule is “Any triplet where the second number is greater than the first and the third greatet than the second”. Original credit to Eliezer in HPmoR but I thought of it because that was what Yvain did at a meetup I was at.

To be clear, since reading this made me double-take, I think by "original credit" you mean "original credit for your personal exposure to the concept".